What is the role of the blood-brain barrier in antihistamine side effects?

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antihistamines, including Benadryl — antihistamine Antihistamines such as Benadryl (diphenhydramine) and Claritin (loratadine) can be purchased without prescriptions. Both of those agents act by blocking H₁ receptors. However, whereas Benadryl binds to those receptors in the central nervous system (CNS), causing drowsiness, Claritin does not readily enter the CNS and thus does not normally cause drowsiness.

The blood-brain barrier plays a critical role in determining the sedative side effects of H1 antihistamines. Older antihistamines are capable of crossing the blood-brain barrier, where they bind to H1 receptors in the central nervous system. This interaction is the primary cause of drowsiness, a common side effect that can impair performance and enhance the effects of other depressant drugs.1

In contrast, newer H1 antihistamines are designed to avoid crossing the blood-brain barrier. By remaining outside the central nervous system, these medications are relatively free of the sedative effects associated with their predecessors.1